All methods of changing left and right must be private or a user in the same package could put it into a state where the validator returns false

The option A. "left, right, setLeft, and setRight must be private" is the correct one to ensure that if p is an instance of Pair, then p.isValid() always returns true. This is because if the fields left and right and the methods setLeft and setRight are private, then they cannot be accessed or modified outside of the Pair class. This means that the only way to modify the left and right fields is through the set method, which already checks the biFunction condition before setting the values of left and right. If the biFunction condition is not met, the set method throws an IllegalArgumentException, so it is not possible to create a Pair instance that is not valid.

The correct answer is C. left and right must be private. This is because the requirement states that if p is an instance of Pair, then p.isValid() must return true. To ensure this, the left and right fields must be private so that they can only be modified by the setLeft and setRight methods, which in turn call the validator function to check if the new values are valid before setting them. This ensures that the isValid method always returns true.

minimal changes will be : C. left and right must be private.